On board the four Cluster spacecraft, the Cluster Ion Spectrometry (CIS) experiment measures the full, three-dimensional ion distribution of the major magnetospheric ions (H⁺, He⁺, He⁺⁺, and O⁺) from the thermal energies to about 40 keV/e. The experiment consists of two different instruments: a COmposition and DIstribution Function analyser (CIS1/CODIF), giving the mass per charge composition with medium (22.5°) angular resolution, and a Hot Ion Analyser (CIS2/HIA), which does not offer mass resolution but has a better angular resolution (5.6°) that is adequate for ion beam and solar wind measurements. Each analyser has two different sensitivities in order to increase the dynamic range. First tests of the instruments (commissioning activities) were achieved from early September 2000 to mid January 2001, and the operation phase began on 1 February 2001. In this paper, first results of the CIS instruments are presented showing the high level performances and capabilities of the instruments. Good examples of data were obtained in the central plasma sheet, magnetopause crossings, magnetosheath, solar wind and cusp measurements. Observations in the auroral regions could also be obtained with the Cluster spacecraft at radial distances of 4–6 Earth radii. These results show the tremendous interest of multispacecraft measurements with identical instruments and open a new area in magnetospheric and solar wind-magnetosphere interaction physics. Key words. Magnetospheric physics (magnetopause, cusp and boundary layers; magnetopheric configuration and dynamics; solar wind - magnetosphere interactions)

In this paper we study a flow burst event which took place during enhanced geomagnetic activity on July 22, 2001, when Cluster was located in the postmidnight magnetotail. The flow burst was associated with a clear dipolarization ahead of the high‐speed part of the predominantly Earthward directed flow. Based on the analysis of the four spacecraft data, we found that a ∼2000 km thick dipolarization front moves Earthward and dawnward with a speed of ∼77 km/s. The plasma before this front is deflected, consistent with the plasma ahead of a localized plasma bubble centered at midnight side being pushed aside by the moving obstacle. The main body of the high‐speed flow is directed mainly parallel to the dipolarization front. These observations indicate that the evolution of the dipolarization front across the tail is directly coupled with the fast flow.

Using the observations of three satellites of Cluster (C1, C3, and C4) during the periods July to October 2001 and July to October 2002, we study 209 active time bursty bulk flows (BBFs), the difference between single satellite observations and multisatellite observations, and the difference among three selection criteria (two about BBFs and one about rapid convection event). Single satellite observations show that the average duration of BBFs selected by the criterion of Angelopoulos et al. is 604 s, while multisatellite observations show that the average duration of BBFs is 1105 s. Single satellite sometimes misses the BBFs. The missing ratio of single satellite is 22.4% for the criterion of Angelopoulos et al. and 44.9 % for the criterion of Raj et al. Therefore the single satellite observations cannot tell the true number of BBFs. The multisatellite observations are more important for the criterion of Raj et al. The single satellite observations also show that 22% of substorms are not accompanied by BBFs, while multisatellite observations show that only 4.5% of substorms are not accompanied by BBFs. Thus it seems possible that all substorms are accompanied by BBFs. The occurrence frequency of RCEs in the central plasma sheet obtained by multisatellites is 12.2%. The occurrence frequency of BBFs in the central plasma sheet is 9.5% for single satellite observations and 19.4% for multisatellite observations. So BBFs may contribute more to the transport of magnetic flux, mass, and energy than what was estimated by previous studies based on single satellite observations.